Beamforming is the spatial steering of an antenna array pattern for directional transmission or reception. FIG. 1A is a basic pictorial diagram of a beamforming antenna array 12 configured for spatial steering of the antenna array pattern for directional transmission or reception. In this example, the system 10 also includes a plurality of user devices 14 in communication with the antenna array 12 and an interfering user 16. As shown in FIG. 1A, beamforming is used to lock onto and increase the signal-to-noise ratio (SNR) of desired links to the user devices 14, while simultaneously placing interferers 16 in nulls. Spatial steering uses no moving parts and is performed by weighting and delaying signals at each antenna element so that the resulting radiation pattern points in a specific direction because of constructive interference. FIG. 1B is a pictorial diagram of a basic beamforming antenna array and transceiver system 20. The system includes a beamforming antenna array 22, a transceiver 24 and an adaptive algorithm 26 that determines the angle of arrival and the appropriate delays. The output of the adaptive algorithm 26 is coupled to the beamforming antenna array 22 for generation of one or more steered beams 28. Multi-beam beamforming presents many additional exciting opportunities and benefits. From the point of view of space division multiple access (SDMA), N independent spatial beams theoretically result in an N-fold increase in network capacity. Alternatively, multiple beams allow for more flexible network management. For example, beams may be directed towards high priority users, such as emergency responders, while still providing network availability to normal users. In imaging, multi-beam beamforming opens the doors to real-time imaging and pattern recognition that is too difficult to realize with single-beam beamformers.
Electrical RF beamformers are commercially available but traditionally suffer from several issues related to bandwidth and scalability. The most well-known issue is beam-squint, where different frequency components are steered in slightly different directions because of the use of non-frequency flat RF components and phase shifters, which are inherently frequency sensitive. Beam-squint can cause unintended interference and unintentional fading of wideband signals; as a result, electrical beamformers are typically narrowband. Multi-beam electrical beamformers also exist, and the current state-of-the-art, which is used for military and satellite communications, achieves about ten simultaneous beams. However, the cost and size of such systems is often prohibitively large for civilian applications. Operating N simultaneous beams using electronics is typically performed by using N parallel and independent beamformer structures; in other words, the hardware for multi-beam electrical beamforming is not scalable.
Photonic beamformers arise as a natural remedy to the problems presented by electronics. Optics provides extremely wide bandwidths and is immune to electromagnetic interference. Photonic beamformers weight and delay signals in the optical domain, using variable optical attenuators and sub-picosecond precision optical true-time delays (TTDs). TTDs are extremely important because they eliminate the frequency-sensitive delay in electrical beamformers that is the source of beam-squint. The sub-picosecond precision also translates to high-resolution steering of the beam. Another advantage of photonics is the ability to seamlessly multiplex many signals together using wavelength division multiplexing (WDM), a technique unavailable to electronics. This is the key to scalable multi-beam photonic beamforming. Whereas a ten-beam electronic beamformer requires ten independent sets of cables and delays, a photonic structure needs only a single WDM-compatible TTD and ten wavelengths of light to multiplex. Currently, there are no multi-beam photonic commercial beamformers on the market. An improved multi-beam photonic beamformer is desirable.